Automatic Transfer Switches used to transfer a critical electronic load from a preferred source to an alternate source that utilize standard relays have been in use for some time. There are two serious technical issues however associated with the use of standard relays that are either detrimental to the relay's longevity (thus reducing the reliability of the transfer switch device) or produce multiples of on-off voltage waveforms that are not acceptable to some of the more sensitive electronic loads (thus increasing the risk of dropping the critical loads).
The present invention makes use of a particular type of relay in an automatic transfer switch that overcomes both of these issues.
Conventional (prior art) designs for relay type automatic transfer switches use two types of standard relays; one relay has Normally Open (or NO) contacts and the other relay has Normally Closed (or NC) contacts with the relay coil de-energized in both cases. FIG. 1A shows such an automatic transfer switch ATS, wherein the NC relay R1 is used to power the critical load L from the primary or preferred source of power S1 and the NO relay R2 is used to connect the load L to an alternate or back-up source S2 when required. In the initial state of the transfer switch ATS as shown in FIG. 1A, the contacts of the NC relay R1 are closed and delivering voltage from the source S1 to the load L while the coil C1 of the relay R1 is de-energized, and the contacts of the NO relay R2 are open while the relay coil C2 is de-energized, but the contacts of the NO relay R2 are selectively closable by energizing the coil C2 to connect the load L to the alternate or back up source S2 if the preferred source S1 fails (goes outside a pre-set range) after coil C1 of R1 is energized to open the contacts of R1 thus completing the transfer operation.
The transfer switch ATS comprises sensing and control logic circuits P that determine if the voltage of the source the unit is connected to (the preferred or primary source S1) is within the desired range set by the user (or, typically, pre-set at the factory). This range of acceptable voltages to the user's critical load L is typically + or −10% to 12% of nominal. Once it is determined by the sensing and control logic circuits P that the source voltage S1 has fallen outside the required range and once the sensing and control logic circuits P also determine that the voltage of the alternate or back-up source S2 is within the acceptable range, then the control logic P of the transfer switch ATS commands the NC relay R1 to open (by energizing coil C1) to disconnect the load L from the primary source S1 and, after a certain amount of time delay, the control logic P also commands the NO relay R2 to close (by energizing coil C2) to connect the critical load L to the alternate or back up source S2, thus avoiding a disruption of the load, as shown in FIG. 1B and referred to herein as a “transferred” state. The total time for sensing of unacceptable voltage and the complete transfer operation (opening of the NC relay R1 and closing of the NO relay R2) is less than 20 milliseconds as established by the power quality industry as the acceptable length of an outage to critical electronic loads if they are to continue operation undisturbed. The purpose of the time delay before closing the contacts of the NO relay R2 is to be sure that the contacts of the NC relay R1 have indeed opened completely so there is no chance of a cross connection between the preferred and alternate sources S1,S2. A cross connection would create a cross current between an already failed primary source S1 and the good alternate source S2, thus causing the alternate source to fail as well and drop the critical load.
In the initial (non-transferred) state (FIG. 1A) of the automatic transfer switch ATS both relay coils C1, C2 are de-energized. This is intended by design because it has been well established that statistically the number one cause of failure in relays is the failure of the relay coil (it can either short or open and in both situations the relay is rendered non-functional and can no longer transfer—i.e., a standard relay with a failed coil will return to or remain in its default state under force of the relay biasing spring). The primary cause of relay coil failures is heat. Heat is generated when the relay coil is energized. It is therefore recognized that in order that the transfer switch product ATS remain reliable and durable to continue protecting the user's critical load, the exposure of the relay coil to heat must be minimized. With conventional (prior art) designs such as that shown in FIGS. 1A and 1B, this means that to the extent possible the NC relay coil C1 or the NO relay coil C2 should not be energized. In the above description of operation it is evident that once the preferred source S1 fails and the transfer switch ATS must transfer, both coils C1,C2 must be energized to make and maintain the transferred state. Typically, the coils C1,C2 are initially energized at twice their normal operating voltage in order to speed up the transfer process to quickly complete the transfer operation. Once the transfer from source S1 to source S2 is complete, the voltage to the relay coils C1, C2 is reduced to the “holding” voltage of the coil so the contacts of the NC relay R1 are held open and the contacts of the NO relay R2 are held closed to keep the user load L running on the alternate source S2 and disconnected from the failed preferred source S1. This transferred state, with both coils C1,C2 energized as shown in FIG. 1B, is a very undesirable state for the product and in time it will lead to degradation of the relay coil insulation and may cause it to deteriorate and ultimately fail.
Once the voltage of the preferred source S1 returns to within the normal range as determined by the voltage sensing circuits P then, after a user pre-set or factory pre-set time delay, the automatic transfer switch ATS will re-transfer from the transferred state (FIG. 1B) back to its normal state (FIG. 1A). In this re-transfer operation the contacts of the NO relay R2, which were closed for the transferred state, open and, after the required delay to ensure disconnection of the source S2 from the load L, the contacts of the NC relay R1, which had been opened for the transfer, close, in each case by de-energizing the coils C1, C2 and allowing the relay springs to move the contacts to their default states. The system is now back to its initial normal state (FIG. 1A), with the load L is being served from the preferred source S1, and with the coils C1, C2 of both relays R1,R2 de-energized. The accumulated time during the times the transfer switch is feeding the load L from the alternate source S2 is detrimental to the reliability and life of both relay coils C1, C2.
A second condition that causes both relay coils C1,C2 to be energized in conventional (prior art) automatic transfer switch devices is if the user elects to use the alternate source S2 as the preferred source for the transfer switch. In automatic transfer switches it is required that the user at his sole discretion be able to select either source S1, S2 as the preferred source for a certain critical load L. This is a requirement because users must be able to balance their loads on their two sources of clean power (typically two battery backed UPS systems). Typically it is desired that approximately one-half of the loads be on the preferred source S1 while the other half are on the alternate source S2. Therefore as many as one-half or more of the switches ATS may be set to feed their respective loads L from the alternate source S2 which once again causes that both relay coils C1,C2 remain energized for those transfer switches configured with the alternate source S2 as the preferred (default) source. Preferred source selection is usually made via a push button control or other means but without the need to swap power connection wires which would require that the load L be shut down. This normal state of being powered from the alternate source S2 is also detrimental to the relay life as both coils C1,C2 must be continuously energized to make the source S2 the preferred (default) source and, thus, both coils C1,C2 are subjected to continuous heat with the above-noted undesirable effects.
Another problem with the use of standard relays in transfer switch applications is “contact bounce”. It is well established that when at the start of a transfer operation the relay coils are first energized by high voltage (usually twice the rated coil voltage) to speed up the transfer, the relay contacts make a solid contact by the added force of the higher voltage which after a short time delay is reduced down to a holding voltage at the rated coil voltage. There is either no contact bounce or only perhaps a single one (which does not affect critical loads) in many, many transfers if the output voltage being provided to the critical load is observed by an oscilloscope, because movement of the contacts is actively induced by the coil. In the automatic retransfer operation that follows, however, the holding voltage is simply removed from the coils C1,C2 to allow the relay contacts to relax back to their original (normal) states under force of a biasing spring in each relay R1,R2. Because there is no high voltage to add contact-moving force to this process and the contacts are simply returning by the small force of the spring in the relays R1,R2 that hold the contacts in NC and NO states respectively, the contacts bounce in the absence of any voltage on the coils C1,C2. This contact bounce causes numerous zero crossings of the load voltage as the contacts make and then break and then make and then break and so on. There can be many bounces before the contacts relax to their NC or NO positions. Critical loads L however can be affected by this repetitive and rapid voltage on-off condition or energizing and de-energizing and can drop. Indeed there have been cases of sensitive loads dropping and causing financial losses to the user. In such cases the users have removed the relay type transfer switches ATS and replaced them with SCR based solid state transfer switches that have no contacts since they are solid state.